This invention relates to boosting a signal-to-interference ratio of a mobile station.
In our discussion, we use the following acronyms:
MSMobile StationBSBase StationCDMACode Division Multiple AccessFDMAFrequency Division Multiple AccessTDMATime Division Multiple AccessSIRSignal to Interference RatioHARQHybrid ARQARQAutomatic Repeat RequestGSMGroupe Speciale Mobile of the EuropeanTelecommunication Standards Institute.AMPSAdvanced Mobile Phone ServiceCDMA 2000Third generation CDMA standard for mobilewireless communication.1xEV-DO 1x Evolution Data Only standard.3GPP2Third Generation Partnership Project 2
In a wireless communication system, limited available resources such as frequency and time are shared among the users of the system. As shown in FIG. 1, in a cellular system 10, a covered area is divided into cells 12, and each cell is served by a base station (BS) 14. To increase capacity for a given frequency spectrum being used, a BS's service area may be further divided into sectors 16, for example, three sectors, using directional antennas, as described in T. S. Rappaport, Wireless Communications. Prentice Hall, 1996. Some cellular systems, such as TDMA (GSM, IS-54) and FDMA (AMPS), use so-called frequency reuse or frequency planning to increase communication capacity.
In a frequency reuse system, the same frequency channels are reused in multiple cells. In FIG. 1, for example, all three cells 12, 13, 15 have sectors 16, 18, 20 that bear a given letter (e.g., the letter “C”) indicating that they use the same frequency channel.
Frequency reuse helps users at cell edges, for example, user 22, located at the edge 24 of a cell to achieve a better signal to interference ratio (SIR).
As more and more carrier frequencies are used for frequency reuse, the SIRs of the users in a given cell get better and the SIR distribution among users of the cell gets more even. However, the spectral efficiency gets lower, which will result in lower total system capacity for a given total spectrum.
CDMA systems such as IS-95A/B, CDMA-2000, and 1xEV-DO incorporate maximal frequency reuse in which neighboring cells use the same carrier frequency, i.e., the reuse factor (defined as the number of frequency channels used)=1. Different codes are used to differentiate different cells. This system yields good spectral efficiency, but the SIR distribution within a cell can be uneven depending on the location of the user.
The SIR of a user at a given location is determined by the locations and configurations (e.g., omni cell or three-sectored cell) of the cells. The SIR in turn determines the instantaneous communication rate of the user.
Recently, 3GPP2 approved a new wireless packet data air interface standard called IS-856, sometimes also referred to as 1xEV-DO. IS-856 provides the capability to support high-speed wireless Internet communication at speeds up to 2.45 Mbit/s using only 1.25 MHz spectrum.
FIG. 2 shows an example, for 1xEV-DO, of the percentage distribution of forward link rates of users who are uniformly distributed geographically within a three-sectored cell. As can be seen from the figure, the lowest and highest rates, 38.4 kbps and 2.4 Mbps, differ by a factor of 64. This large difference in rate makes it hard to achieve an even throughput to all users in a cell, as is required for constant bit-rate applications such as voice. For data applications, a certain degree of unfairness, e.g., giving higher throughput for users who are close to the BS and lower throughput for users who are far from the BS, is allowed as long as it does not violate certain fairness conditions.
Because the forward link of 1xEV-DO is TDMA, it is possible to allocate different amounts of time for each user to increase the fairness, i.e., by giving more time slots for low SIR users and fewer time slots for high SIR users. However, this will lower the throughput of the overall system because low SIR users will consume a large share of the resources. Systems designed to increase fairness tend to reduce sector throughput.